When operating an engine with a gaseous fuel it is a well known practice to introduce the gaseous fuel with the inlet air to the cylinder during the air inlet stroke, thus causing a relatively homogenous mixture of gas and air during the combustion stroke. It is advantageous to reduce the fuel/air ratio to a minimum value in the combustion of gas as a fuel, as this reduces the carbon monoxide and hydrocarbon content of the exhaust and enhances fuel efficiencies. If the ratio is reduced sufficiently beyond the stoichiometric value a substantial reduction in the nitrous oxide content of the exhaust gas may also be achieved. This is commonly called a lean burn regime. With current gas engines the limiting factor in the reduction of the ratio to achieve lean burn is that which occurs when the average ratio of fuel to air is lowered to a level where the density of the fuel is such as to prevent initiation of fuel ignition, and thus the engine misfires.